The present invention is directed to catalyst particles comprising a layered core-shell structure (more specifically defined as a core-shell-shell (“c/s/s”) structure) and to a method of their manufacture. The advanced core/shell type particles contain precious metals and find use in various catalytic applications, for example in the preparation of gas-phase catalysts, for the manufacture of electrocatalysts for fuel cells or for the manufacture of catalytic converters for automobiles. They may also be useful in variety of other applications such as, e.g., in electronic or medical applications.
Electrocatalysts for fuel cells are generally based on nanoparticles of platinum or platinum alloys (medium particle size <10 nm), supported on high surface area support materials (e.g. carbon blacks). Carbon supported PtCo or PtNi alloy catalysts are currently used in polymer electrolyte membrane (PEM) fuels cells for mobile applications. To become commercial competitive it is necessary to further reduce the platinum consumption and at the same time to improve the activity and the lifetime of such platinum based catalysts, in particular for mobile PEMFC technology.
It is well known in the art, that only the surface of a catalyst particle promotes the catalytic reaction and the inside core does not contribute to its reactivity. Thus, it should be possible to further reduce the amount of platinum by having it present in a shell at the particle surface. Catalysts of this type are known as core-shell catalysts. Over the past years, such electrocatalysts were described in the literature.
The core-shell-type catalyst particles such as, for example, Pt coated Ni particles (hereinafter abbreviated Ni@Pt for short) or Pt-coated Co particles (Co@Pt), gain increased importance and find use as catalysts for fuel cells or electrode materials for batteries. Especially the particles with a Pt-based shell show a high specific activity. As an advantage, such particles provide a low precious metal content due to the core-shell structure. The catalyst particles are characterized by a high specific mass activity (“SMA”) and an improved performance in oxygen reduction reactions (“ORR”) at the cathode of PEMFCs or DMFCs (Direct methanol fuel cells).
J. Zhang et al. reported the preparation of core-shell particles as electrocatalysts for oxygen reduction. The core comprises of an alloy of a precious metal, whereas the shell consists of a Pt monolayer deposited by under potential deposition (“UPD”); ref to J. Zhang, F. H. B Lima et al, Journal of Physical Chemistry B Letters, 2005, 109, 22701-22704. The catalyst thus obtained is a PtMLX/C (X═Au, Ag, Pd; ML=monolayer) with the metal particles comprising an inner core consisting of metal X and a monolayer of platinum in form of a shell on top of it.
Core-shell catalysts comprising a ruthenium core coated with platinum (Ru@Pt) were described some years ago (ref to S. R. Brankovitch, J. X. Wang and R. R. Adzic, Electrochemical and Solid State Letters 2001, 4, A 217). The medium particle size of the Ru@Pt core/shell particles is in the range of 2.5 nm (by TEM).
U.S. Pat. No. 7,053,021 teaches the preparation of carbon-supported core-shell nano-particles of 1-3 nm size comprising a platinum-vanadium-iron alloy. An improvement by the factor of 2-4 is reported. However, this improvement is not sufficient to meet the targets of the auto industry.
Examples for core/shell-type catalysts, primarily for use as electrocatalysts in fuel cells are disclosed in U.S. Pat. No. 8,227,372, U.S. Pat. No. 8,288,308, U.S. Pat. No. 8,304,362 and US 2012/0316054 to the same applicant. These core/shell particles comprise a metal or ceramic core material and at least three atomic layers of platinum in their shell.
WO2012/123442A1 (also published as US2012/0238443A1) to the same applicant is directed to a method for manufacture of metal nanoparticles, in particular to the manufacture of nano-sized base metal particles. The manufacturing method of this invention is based on the “seed particle method” or “seed-mediated method”. By this method, size-controlled base metal particles with a medium particle diameter in the range of 20 to 200 nm can be produced using small precious metal seed particles (“nuclei”) to initiate the particle formation. Such particles are preferably used as starting material in the present invention.
In summary, the electrocatalyst presently state of the art are not sufficient to meet the performance and cost requirements required for the widespread commercial introduction of fuel cell technology.
It is one objective of the present invention to provide improved core-shell type electrocatalyst materials showing a continuous layer of Pt (or Pt-based alloy) at the outside surface. As a result, a high performance at low platinum loadings, particularly in automotive fuel cell applications should be feasible.
It is a further objective to provide a method for the preparation of such core-shell type catalyst materials. The method should be based on a simple and economic synthesis route; it should be environmentally safe and should be easily scaleable for industrial production.